The present invention relates to a sound-insulating device of the kind described in the preamble to the independent claim 1. The invention also relates to a liquid-insulated induction machine of the kind described in the preamble to the independent claim 14.
In this patent application, induction machine means a stationary induction machine,. that is, a transformer or an inductor. More particularly, the invention relates to a transformer. or an inductor-for voltage exceeding 1 kilovolt for a distribution or a transmission network.
A liquid-insulated induction machine comprises a tank, filled with insulating fluid, in which an active part is placed. In this connection, active part means an iron core and a winding subassembly. Due to electromagnetic forces, the active part oscillates during operation. These oscillations propagate in the insulating fluid to the roof, bottom and wall portions of the tank, which portions, outside the tank, generate an audible sound which may attain such sound intensities that it constitutes a problem. This is. particularly the case for induction machines placed in densely populated areas.
It is known to reduce the above-mentioned sound by placing, between the active part and the tank, a sound-insulating device comprising a gas-filled cavity, for the purpose of preventing oscillations in the insulating fluid from reaching the floor, bottom or wall portions of the tank. However, known sound-insulating devices have a limited compressibility, which has proved to suppress the sound-damping effect.
U.S. Pat. No. 1,846,887 describes a sound-insulating device of the type described above, in which a hollow, gas-filled double wall with rigid spacing blocks is placed between the active part of a transformer and the tank thereof. The task of the double wall is to absorb oscillations generated by the active part and to prevent these oscillations from reaching the tank. However, the rigid spacing blocks limit the compressibility of the double wall and convey the oscillations from one side of the double wall to the other side thereof, whereby the oscillations easily pass through the double wall.
Another sound-damping device of the type described above is described in U.S. Pat. No. 4,558,296 in the form of a sound-damping plate which is attached to the inside of a transformer tank. The plate has a front wall, a side wall and a rear wall which define a gas-filled cavity. The front wall has a frame-shaped edge portion, extending along the major part of its circumference, the average wall thickness of the edge portion being considerably smaller than the average total wall thickness of the front wall. Admittedly, by the relatively thin edge portion, the plate exhibits a limited compressibility, but the rigid mid-portion of the front wall reduces the same and suppresses the sound-damping ability of the plate. In addition, the location of the plate directly on the inside of the tank causes vibrations to be easily transmitted from the plate to the tank.
The object of the invention, from a first aspect of the invention, is to achieve a new type of sound-insulating device which is extremely compressible and which, at the same time, is simple in its construction, easy to manufacture and durable. This is achieved-according to the invention by a sound-insulating device according to the features described in the characterizing portion of the independent claim 1.
The object of the invention, from a second aspect of the invention, is to achieve an efficiently sound-damped stationary induction machine. This is achieved according to the invention by an induction machine according to the features described in the characterizing portion of the independent claim 14.
Advantageous embodiments are described in the characterizing portions of the dependent claims.
Experiments have shown that, in an induction machine with an active part, an insulating fluid surrounding the active part, and a tank enclosing the insulating fluid, an efficient sound insulation may be achieved by a sound-insulating device which, in contrast to known sound-insulating devices, is extremely compressible and resilient to all sound-generating oscillations occurring in the fluid, which sound-insulating device is placed between the active part and the tank, and spaced from the inside of the tank. The present invention aims to provide such a device.
The sound-insulating device according to the invention comprises a gas-filled cavity and a resilient membrane surrounding the cavity. The task of the membrane is to give the cavity a desired shape, to keep the cavity at the desired location in the induction machine, and to prevent the gas in the cavity from mixing with the insulating fluid. Within the frameworks which these tasks mechanically impose on the membrane, the membrane shall be as resilient as possible. In this context, it is very important for the gas not to leak out into the insulating fluid, since the insulating effect of the fluid in that case would be greatly deteriorated, which may result in damage to the induction machine.
The sound-insulating device preferably has an extent in one plane. In an induction machine, the sound-insulating device is arranged such that this plane substantially forms a right angle with the direction of propagation of the oscillations. The sound-insulated device thus has a first membrane portion which substantially faces the active part and a second membrane portion which is arranged in parallel with the first membrane portion and which substantially faces the inside of the tank.
In its simplest and most resilient embodiment, the membrane consists of rubber or some other polymer material. An induction machine may, however, have a service life of more than 30 years. Therefore, from the point of view of strength, a membrane of thin sheet metal is preferable to a polymer membrane since the sound-insulating device must operate during the whole life of the induction machine without the gas in the cavity leaking out. According to a preferred embodiment, the membrane is made from thin, stainless sheet steel, preferably of uniform thickness. From such a sheet, a membrane may be manufactured in a simple and rational way, which membrane is very resilient but which at the same time makes it possible to form the sound-insulating device into the desired shape. Preferably, the sound-insulating device is made from two thin sheets which are pressed and which, along their edges, are-gas-tightly attached to each other so as to surround the above-mentioned cavity. The sheets thereby form two membrane halves with an intermediate gas volume.
A sound-insulating device mounted in an induction machine, filled with insulating fluid, is influenced by the atmospheric pressure plus the hydrostatic pressure of the fluid, which gives an absolute pressure of about 100-200 kPa, depending on whether the sound-insulating device is placed at a high or a low level in the tank of the induction machine. The sound-insulating device must be able to withstand this pressure without the membrane being. compressed to such an extent that opposite membrane portions are brought into rigid contact with one another, in which case the sound-insulating ability of the device would be greatly deteriorated.
According to one embodiment of the sound-insulating device, the pressure in the cavity is equal to or higher than the absolute pressure of the insulating fluid. However, a high pressure in the cavity suppresses the sound-insulating compressibility of the device, and preferably the pressure in the cavity shall be as low as possible without the opposite membrane portions being brought into rigid contact with one another.
According to another embodiment of the sound-insulating device, the pressure in the cavity is lower than the absolute pressure of the insulating fluid, and a resilient spacing member is arranged in the cavity-making contact with the membrane at at least two points. The spacing member prevents rigid contact between opposite membrane portions, whereby a low pressure may be allowed in the cavity.
According to a further embodiment of the sound-insulating device, at least one region of the membrane is folded or corrugated, whereby a membrane is obtained which withstands the pressure from the insulating fluid but which, at the same time, is resilient to oscillations in the fluid. In a membrane of thin sheet, folding may be easily achieved by pressing the sheet when manufacturing the sound-insulating device.
To obtain a good sound-insulating effect, it is advantageous that the sound-insulating device is not placed in direct contact with the inside of the tank. Insulating fluid should occur between the sound-insulating device and the inside of the tank. Experiments have shown that it is advantageous to place the sound-damping device closer to the active part than the inside of the tank, and according to a preferred embodiment of the induction machine, the sound-insulating device is placed such that the shortest distance between the device and the active part is smaller than the shortest distance between the sound-insulating device and the inside of the tank. Preferably, the sound-insulating device is placed as close to the active part as possible, whereby the liquid volume between the sound-damping plate and the inside of the tank is as large as possible.